Photolithographically patterned spring contact

ABSTRACT

A photolithographically patterned spring contact is formed on a substrate and electrically connects contact pads on two devices. The spring contact also compensates for thermal and mechanical variations and other environmental factors. An inherent stress gradient in the spring contact causes a free portion of the spring contact to bend up and away from the substrate. An anchor portion remains fixed to the substrate and is electrically connected to a first contact pad on the substrate. The spring contact is made of an elastic material and the free portion compliantly contacts a second contact pad, thereby electrically interconnecting the two contact pads. The free portion is initially fixed to the substrate to intentionally form the inherent stress gradient in the elastic member. The free portion is released from the substrate by etching a release layer deposited on the substrate so the inherent stress gradient in the elastic member biases the free portion away from the substrate. A contact tip of the spring contact can be coated with solder.

This is a continuation of application Ser. No. 09/210,552 filed Dec. 14,1998, which in turn is a continuation of application Ser. No. 08/770,285filed Dec. 20, 1996 (now issued as U.S. Pat. No. 5,848,685), which inturn is a continuation of application Ser. No. 08/478,578 filed Jun. 7,1995 (now issued as U.S. Pat. No. 5,613,861). The entire disclosures ofthe prior applications are hereby incorporated by reference herein intheir entirety.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention generally relates to photolithographically-patternedspring contacts for use in electrically bonding integrated circuits,circuit boards, electrode arrays, or other devices.

2. Description of Related Art

Standard bonding techniques for electrically connecting integratedcircuits, or chips, to a circuit board or other device include wirebonding, tab bonding, solder-bump and gold-bump flip-chip bonding andother techniques. FIG. 1 shows a contact pad 3 formed on a chip 2 wirebonded to a corresponding contact pad 3 formed on a substrate 1. Thecontact pads 3 are electrically connected, or bonded, by a wire 4. Sincethe chip 2 typically has tens or even hundreds of the contact pads 3,wire bonding each contact pad 3 on the chip 2 to the correspondingcontact pad 3 on the substrate 1 is labor intensive, expensive and slow.Further, the contact pads 3 must be large enough to accommodate both thewire 4 and the accuracy of the wire bonding device used to create thewire bond. Therefore, the contact pads 3 are made larger than otherwisenecessary to compensate for the size limitations of the wire 4 and thewire bonding device.

FIG. 2 shows the contact pad 3 formed on the chip 2 tab bonded to thecorresponding contact pad 3 on the substrate 1. A flexible substrate 5having conductive lines formed on its lower surface is forced againstthe contact pads 3. A layer of anisotropic adhesive (not shown) isplaced between the contact pads 3 and the flexible substrate 5. When theflexible substrate 5 is pressed against the contact pads 3, theanisotropic adhesive and the conductive lines formed on the flexiblesubstrate 5 cooperate to complete the electrical connection between thecontact pads 3. Like wire bonding, tab bonding suffers from yield loss,bond fragility, and high cost.

Another conventional method for bonding the contact pads 3 formed on thechip 2 to the contact pads 3 formed on the substrate 1 or to some otherdevice is solder-bump flip-chip bonding. FIG. 3 shows the chip 2inverted with the contact pads 3 facing toward the substrate 1. The name“flip-chip” derives from the inversion of the chip 2, since the chip 2is “flipped over” with the contacts pads 3 facing the substrate 1, incontrast to both tab bonding and wire bonding where the contact pads 3on the chip 2 face away from the substrate 1. Note, however, that tabbonding can also be done with the chip 2 “flipped over”. In standardflip-chip bonding, solder bumps 6 are formed on the contact pads 3 onthe substrate 1 or on the chip 2. The electrical connection between thecorresponding contact pads 3 is completed by pressing the contact pads 3on the chip 2 against the solder bumps 6 and melting the solder bumps 6.

Flip-chip bonding is an improvement over both wire bonding and tabbonding. The relatively soft solder bumps 6 tend to permanently deformwhen the chip 2 is pressed down against the solder bumps 6. Thisdeformation of the solder bumps 6 compensates for some irregularity inthe heights of the contact pads 3 and any uneven contacting pressureforcing the chip 2 against the solder bumps 6.

However, flip-chip bonding does suffer from both mechanical and thermalvariations in the solder bumps 6. If the solder bumps 6 are not uniformin height or if the substrate 1 is warped, contact between the contactpads 3 and the solder bumps 6 can be broken. Also, if the contactingpressure forcing the chip 2 down on the solder bumps 6 is uneven,contact between some contact pads 3 and corresponding solder bumps 6 canfail. In addition, stresses from thermal expansion mismatches betweenthe chip 2 and the substrate 1 can break the bonds formed by the solderbumps 6.

In contrast to the relatively permanent bonds described above, FIG. 4shows a standard technique for establishing a temporary electricalcontact between two devices. A probe card 7 having a plurality of probeneedles 8 contacts the contact pads 3 by physically pressing the probeneedles 8 against the contact pads 3. The physical contact between theprobe needles 8 and the contact pads 3 creates an electrical connectionbetween the probe needles 8 and the lines 9 formed on the substrate 1.

The probe cards 7 are generally used to create only temporary contactsbetween the probe needles 8 and the contact pads 3, so that the device10 can be tested, interrogated or otherwise communicated with. Thedevice 10 can be a matrix of display electrodes which are part of anactive-matrix liquid crystal display. Testing of the devices 10, such asliquid crystal display electrode matrices, is more thoroughly describedin a U.S. patent application Ser. No. 8/473,912, now abandoned to thesame inventor, co-filed and co-pending herewith.

The probe cards 7 have many more applications than only for testingliquid crystal displays. Any device 10 having numerous and relativelysmall contact pads 3, similar to those found on the chip 2, can betested using the probe card 7. However, standard techniques forproducing the probe card 7 are time consuming and labor-intensive. Eachprobe card 7 must be custom-made for the particular device 10 to betested. Typically, the probe needles 8 are manually formed on the probecard 7. Because the probe cards 7 are custom-made and relativelyexpensive, the probe cards 7 are not typically made to contact all ofthe contact pads 3 on the device 10 at one time. Therefore, onlyportions of the device 10 can be communicated with, tested orinterrogated at any one time, requiring the probe card 7 be moved toallow communication, testing or interrogation of the entire device 10.

The probe cards 7 are also used to test the chips 2 while the chips 2are still part of a single-crystal silicon wafer. One such probe card 7is formed by photolithographic pattern plated processing, as disclosedin Probing at Die Level, Corwith, Advanced Packaging, February, 1995,pp. 26-28. Photolithographic pattern plated processing produces probecards 7 which have essentially the same design as the standard probecard 7. However, this new type of processing appears to automate themethod for producing probe needles 8, thus avoiding manually forming theprobe needles 8. Also, this article discloses a probe card 7 which isbent at the end nearest the probe needles 8, as shown in FIG. 5. Thebend in the probe card 7 allows the probe needles 8 to contact thecontact pad 3 at an angle. As the probe card 7 pushes the probe needles8 into the contact pads 3, a mechanical scrubbing action occurs whichallows the probe needles 8 to break through the oxide formed on the topsurface of the contact pad 3. All of the standard probe cards 7,however, are limited to testing contact pads 3 which are arranged in alinear array.

SUMMARY OF THE INVENTION

Accordingly, this invention provides a spring contact which exhibits thespeed and ease of solder-bump flip-chip bonding while eliminating theneed to create uniform solder bumps or uniform contacting pressure.Also, the invention provides finer-pitch contact arrays than solder-bumpflip-chip bonding.

This invention further provides a spring contact which has elasticproperties enabling the spring contact to maintain physical contact witha contact pad despite variations in contact pad heights, contactingpressure, thermal variations or mechanical shock.

This invention also provides an elastic spring contact having a stressgradient formed in the spring contact, which causes the spring contactto bend away from the substrate and thus provide compliant contact witha contact pad.

This invention further provides a probe card and a method for producingthe probe card having spring contacts in place of standard probeneedles.

The spring contacts of this invention are formed of a thin metal stripwhich is in part fixed to a substrate and electrically connected to acontact pad on the substrate. The free portion of the metal strip notfixed to the substrate bends up and away from the substrate. When thecontact pad on a device is brought into pressing contact with the freeportion of the metal strip, the free portion deforms and providescompliant contact with the contact pad. Since the metal strip iselectrically conductive or coated with a conductive material, thecontact pad on the substrate is electrically connected to the contactpad on the device via the spring contact.

BRIEF DESCRIPTION OF THE DRAWINGS

This invention will be described in relation to the following drawings,in which reference numerals refer to like elements and wherein:

FIG. 1 shows a chip wire bonded to a substrate;

FIG. 2 shows the chip tab bonded to the substrate;

FIG. 3 shows the chip solder-bump flip-chip bonded to the substrate;

FIG. 4 shows a probe card contacting an electronic device;

FIG. 5 shows a probe card having an angled probe needle;

FIG. 6 is a spring contact in an undeformed free state and anotherspring contact deformed when contacting a contact pad;

FIG. 7 shows a metal strip with no stress gradient;

FIG. 8 shows a model for determining the curvature of a spring contactdue to the stress gradient;

FIG. 9 shows a model for determining the amount of reaction forceexerted at the tip of the spring contact;

FIG. 10 shows the first steps in a method of forming a spring contactaccording to the invention;

FIG. 11 shows additional steps following those shown in FIG. 10 in amethod of forming a spring contact according to the invention;

FIG. 12 shows additional steps following those shown in FIG. 11 in amethod of forming a spring contact according to the invention;

FIG. 13 shows a final step in a method of forming a spring contactaccording to the invention;

FIG. 14 is a graphic representation of the film stress in a sputterdeposited nickel-zirconium alloy as a function of plasma gas pressure;

FIG. 15 is a top view of a spring contact;

FIG. 16 is a device for testing the contact resistance of a plurality ofspring contact pairs;

FIG. 17 is a graphical representation of the detected resistance of aplurality of spring contact pairs;

FIG. 18 is a graphic representation of the contact resistance of aspring contact as a function of the distance between the contact pad andthe substrate;

FIG. 19 is a spring contact having a flat end;

FIG. 20 is a spring contact having a pointed end;

FIG. 21 is a spring contact having two points at the tip end;

FIG. 22 is a spring contact having multiple points at the tip end;

FIG. 23 is a spring contact having a deformable tab at the tip end;

FIG. 24 shows a spring contact having a deformed tab end when forcedagainst a contact pad;

FIG. 25 is a chip having a plurality of spring contacts electricallybonded to a substrate;

FIG. 26 is a chip bonded to a dust cover and electrically contacted to asubstrate having a plurality of spring contacts;

FIG. 27 is a chip bonded to a substrate and electrically contacted to asubstrate by a plurality of spring contacts on the chip;

FIG. 28 is a chip electrically bonded to a substrate by way of anintermediate wafer having a plurality of spring contacts;

FIG. 29 is a probe card having a plurality of spring contacts used fortesting an electronic device; and

FIG. 30 is a liquid crystal display and a device for testing theoperation of the display.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

FIG. 6 shows a side view of a bonding structure 100 having a pluralityof spring contacts 15. Each spring contact 15 comprises a free portion11 and an anchor portion 12 fixed to an insulating underlayer 13 andelectrically connected to a contact pad 3. Each spring contact 15 ismade of an extremely elastic material, such as a chrome-molybdenum alloyor a nickel-zirconium alloy. Preferably, the spring contacts 15 areformed of an elastic conductive material, although they can be formed ofa non-conductive or semi-conductive material if they are coated orplated with a conductor material. More preferably, the spring contacts15 are formed of a nickel-zirconium alloy having 1% zirconium. Zirconiumis added to the nickel to improve the elastic properties of the alloywhile not greatly reducing the conductivity of the nickel. When theelastic material is not conductive, it is coated on at least one sidewith a conductive material, such as a metal or metal alloy.

The contact pad 3 is the terminal end of a communication line whichelectrically communicates with an electronic device formed on thesubstrate 14 or device 101 such as a transistor, a display electrode, orother electrical device. The contact pad 3 is typically made ofaluminum, but can be made of any conductive material. If the contact pad3 on device 101 is made of aluminum, the contact pad 3 is preferablycoated with a conductive material, such as gold, indium tin oxide, ornickel. This allows the spring contact 15 to make better electricalcontact with the contact pad 3, since the spring contact 15 cannot“scrub” the uncoated contact pad 3 to break through the aluminum oxidethat forms on an uncoated aluminum contact pad 3. The insulatingunderlayer 13 is made of silicon nitride or other etchable insulatingmaterial. However, the insulating underlayer 13 is not necessary and canbe eliminated. The insulating underlayer 13 and the contact pad 3 areformed on or over a substrate 14, which is also formed of an insulatingmaterial, such as oxidized silicon or glass.

As shown in FIG. 7, a strip of metal having no stress gradient inherentin the metal will lie flat. However, as shown in FIG. 8, when the stripis bent into an arc, a uniform stress gradient Δσ/h is introduced intothe strip. Likewise, if a uniform stress gradient Δσ/h is introducedinto the flat metal strip, the metal strip will bend into an arc shape.

Each spring contact 15 is formed such that a stress gradient Δσ/h isintroduced into the spring contact 15. When the spring contact 15 isformed, the metal layer comprising the spring contact 15 is depositedsuch that compressive stress is present in upper portions of the metallayer and tensile stress is present in lower portions of the metallayer. Compressive stress in upper portions of the metal layer isdepicted by arrows directed inwardly. Tensile stress is depicted inlower portions of the metal layer by arrows directed outwardly. Thestress gradient Δσ/h causes the spring contact 15 to bend into the shapeof an arc having a radius r. Equation 1 gives the radius of curvature rof the spring contact 15: $\begin{matrix}{r = {( \frac{Y}{1 - v} )\frac{h}{\Delta\sigma}}} & (1)\end{matrix}$

where Y is the Young's modulus of the metal, h is the thickness of themetal layer forming the spring contact 15, Δσ is the total stressdifference, and v is the Poisson's ratio of the metal.

Referring again to FIG. 6, r is the radius of curvature of the freeportion 11 of the spring contact 15 as predicted in Equation 1, and Θ isthe angle separating the radius line directed toward the junction of thefree portion 11 with the anchor portion 12 and the radius line directedtoward the tip 30 of the free portion 11. Equation 2 gives theapproximate height b of the spring contact tip 30 from the substrate 14for angles Θ<50°: $\begin{matrix}{b \approx \frac{L^{2}}{2r}} & (2)\end{matrix}$

where L is the length of the free portion 11 and r is the radius ofcurvature of the free portion 11.

Since each spring contact 15 is preferably made of a highly elasticmaterial, each spring contact 15 can be pushed down at the tip 30 anddeformed as shown in FIG. 6, but will not plastically deform. Typically,a contact pad 3 of a device 101 exerts the downward force placed on thetip 30 and electrically contacts the tip 30. The spring contact 15resists the downward force placed on the tip 30 and maintains electricalcontact with the contact pad 3.

When the force on the tip 30 is released, the spring contact will returnto its undeformed state. Thus, the elasticity of the spring contacts 15allows the spring contacts 15 to make numerous successive electricalconnections with different contact pads 3 while maintaining theintegrity of the electrical connection between the spring contact tip 30and the contact pads 3.

Additionally, the spring contact 15 is preferably made of acreep-resistant material. Therefore, when the spring contact 15 iselastically deformed over an extended period by a contact pad 3 pressingdown on the spring contact tip 30, the spring contact 15 resists thedownward force and pushes the spring contact tip 30 against the contactpad 3, maintaining the electrical connection.

FIG. 9 shows a model for determining the amount of force F_(tip) appliedby the spring contact tip 30 to a contact pad 3 in reaction to the forceof the contact pad 3 pressing down on the spring contact tip 30.Equation 3 gives the reaction force F_(tip) of the spring contact tip30: $\begin{matrix}{F_{tip} = \frac{{wh}^{2}{\Delta\sigma}}{12x}} & (3)\end{matrix}$

where w is the width of the spring contact 15, h is the thickness of thespring contact 15, Δσ is the total stress difference and x is thehorizontal distance from the spring contact tip 30 to the point wherethe spring contact 15 first touches the substrate 14.

For a given width w, thickness h and stress difference Δσ, the reactionforce F_(tip) of the tip 30 varies inversely with the distance x.Therefore, the reaction force F_(tip) increases as the spring contacttip 30 gets closer to the substrate 14, since the distance x decreasesas the spring contact 15 collapses and presses against the substrate 14as shown in FIG. 6. The increase in the reaction force F_(tip) as thecontact pad 3 presses the spring contact tip 30 closer to the substrate14 generally improves the electrical connection between the springcontact tip 30 and the contact pad 3. The increasing reaction forceF_(tip) causes the spring contact tip 30 and/or the contact pad 3 todeform locally at the area of contact, increasing the area of contactbetween the contact pad 3 and the spring contact tip 30.

FIGS. 10-13 show the basic steps in forming a spring contact 15. In FIG.10, a contact pad 3 is formed on or over a substrate 14. Additionally,an insulating underlayer 13 is formed on or over the substrate 14.However, as mentioned above, the insulating underlayer 13 is notrequired and can be eliminated.

In FIG. 11, a layer of metal 16 is deposited on or over the substrate14. In the preferred embodiment of the invention, the metal is thenickel-zirconium alloy described above. Part of the metal layer 16 iselectrically connected to or directly contacts the contact pad 3 andanother portion of the metal layer 16 is deposited on or over theinsulating underlayer 13. There are many methods available fordepositing a metal layer 16 on or over the substrate 14, includingelectron-beam deposition, thermal evaporation, chemical vapordeposition, sputter deposition and other methods. Preferably, the metallayer 16 is sputter deposited.

When sputter-depositing a metal, a plate of the metal, called thetarget, is placed on a cathode, which is set to a high negativepotential and immersed in a low-pressure, typically 1 to 100 millitorr,gas. This causes a glow-discharge plasma to ignite, from which positiveions are accelerated into the negatively charged target. This ionbombardment knocks metal atoms off the target, and many of these depositon nearby surfaces, such as the substrate 14.

The metal layer 16 can be thought of as deposited in several sub-layers16-1 to 16-n to a final thickness h of approximately 1 μm. The stressgradient Δσ/h is introduced into the metal layer 16 by altering thestress inherent in each of the sub-layers 16-1 to 16-n of the metallayer 16, as shown in FIG. 11, each sub-layer 16-x having a differentlevel of inherent stress.

Different stress levels can be introduced into each sub-layer 16-x ofthe deposited metal layer 16 during sputter deposition in a variety ofways, including adding a reactive gas to the plasma, depositing themetal at an angle, and changing the pressure of the plasma gas.Preferably, the different levels of stress are introduced into the metallayer 16 by varying the pressure of the plasma gas, which is preferablyargon.

FIG. 14 is a graph showing a typical relationship of the film stress inthe sputter deposited nickel-zirconium alloy and the pressure of theplasma gas used in the deposition. For low pressures of the plasma gas,approximately 1 mTorr, the film stress in the deposited metal iscompressive. As the pressure of the plasma gas increases, the filmstress in the deposited sub-layer changes to a tensile stress andincreases with increasing plasma gas pressure.

Preferably, the metal layer 16 is deposited in five sub-layers 16-1 to16-5. The first sub-layer 16-1 is deposited at a plasma gas pressure of1 mTorr, as indicated by numeral 1 in FIG. 14. The first sub-layer 16-1is the bottom-most layer in the metal layer 16 and has an inherentcompressive stress. The second sub-layer 16-2 is deposited on top of thefirst sub-layer 16-1 at a plasma gas pressure of approximately 6 mTorr.The second sub-layer 16-2 has a slight inherent tensile stress, asindicated by numeral 2 in FIG. 14. Sub-layers 16-3, 16-4 and 16-5 arethen deposited one on top of the other at the plasma gas pressuresindicated by numerals 3, 4 and 5 in FIG. 14.

The process of depositing the metal layer 16 in five separate sub-layers16-1 to 16-5 results in the metal layer 16 having a stress gradient Δσ/hwhich is compressive in the lower portion of the metal layer 16 andbecomes increasingly tensile toward the top of the metal layer 16.Although the stress gradient Δσ/h urges the metal layer 16 to bend intoan arc, the metal layer 16 adheres to the insulating underlayer 13, thesubstrate 14 and the contact pad 3 and thus lies flat.

After the metal layer 16 is deposited, the metal layer 16 isphotolithographically patterned into the spring contacts 15.Photolithographic patterning is a well-known technique and is routinelyused in the semiconductor chip industry. First, a positivephotosensitive resist 17 is spun on top of the metal layer 16 andsoft-baked at 90° C. to drive off solvents in the resist 17. Thephotosensitive resist 17 is exposed to an appropriate pattern ofultra-violet light and then developed. Exposed areas of the resist 17are removed during developing and the remaining resist 17 is hard-bakedat 120° C. Wet or plasma etching is then used to remove the exposedareas of the metal layer 16. The remaining areas of the metal layer 16after etching form the spring contacts 15. A top-view of one springcontact 15 is shown in FIG. 15. The area of the metal layer 16 removedby the etching is described by the dashed line 18.

Next, as shown in FIG. 12, the free portion 11 of the spring contact 15is released from the insulating underlayer 13 by a process of under-cutetching. Until the free portion 11 is released from the insulatingunderlayer 13, the free portion 11 adheres to the insulating underlayer13 and the spring contact 15 lies flat on the substrate 14. There aretwo methods for releasing the spring contacts 15 from the substrate 14or insulating underlayer 13. In the first method, the insulatingunderlayer 13, typically silicon nitride, is deposited by plasmachemical vapor deposition (PECVD) at a temperature of 200-250° C. Thisgives the insulating underlayer 13 a fast etch rate. The insulatingunderlayer 13 is then pre-patterned, before the metal layer 16 isdeposited, into islands on which the spring contacts 15 will be formed.After the spring contacts 15 are formed on or over the islands of theinsulating underlayer 13, the spring contacts 15 are released from theinsulating underlayer 13 islands by etching the islands with a selectiveetchant. The selective etchant is typically a HF solution. The etchantis called a selective etchant because it etches the insulatingunderlayer 13 faster than the selective etchant removes metal from thespring contacts 15. This means that the spring contacts 15 are releasedfrom the insulating underlayer 13 and are allowed to bend up and awayfrom the insulating underlayer 13 due to the stress gradient Δσ/h in thespring contacts 15. The islands can also be formed of a low meltingtemperature material, such as solder or plastic. After the springcontacts 15 are formed, the low melting temperature material is heatedto release the spring contacts 15.

In the second method for releasing the spring contacts 15, theinsulating underlayer 13, if used, is not pre-patterned into islands.Instead, after the spring contacts 15 are formed, a passivating layer,such as silicon oxynitride, is deposited on the spring contacts 15 andthe surrounding areas by PECVD. The passivation layer is patterned intowindows, such as the shaded area shown in FIG. 15, to expose the freeportion 11 of the spring contacts 15 and surrounding areas of theinsulating underlayer 13. The same selective etchant, the HF solution,is used to etch the insulating underlayer 13 and release the springcontacts 15. This method avoids a step discontinuity in the metal of thespring contact 15 at the anchor portion 12 edge and leaves an insulatingcover (not shown) on the anchor portion 12. The insulating coverprotects the anchor portion 12 from short-circuiting and also helps holdthe anchor portion 12 down on the substrate 14.

Only those areas of the insulating underlayer 13 under the free portion11 of the spring contact 15 are under-cut etched. The area of insulatingunderlayer 13 under-cut etched for each spring contact 15 is describedby the shaded portion in FIG. 15. This means that the anchor portion 12of the spring contact 15 remains fixed to the insulating underlayer 13and does not pull away from the insulating underlayer 13. It should beappreciated that the method for patterning the metal layer 16 into thespring contacts 15 should not result in any annealing of the metal layer16.

Additional steps can be added to the under-cut etching processes toimprove the processes if necessary. For example, etchant vias, or smallwindows, can be etched into the free portions 11 of the spring contacts15. The etchant vias operate to provide the selective etchant fasteraccess to the insulating underlayer 13, thereby speeding the process ofreleasing the free portions 11 from the insulating underlayer 13. Also,a hard mask, made of, for example, silicon, can be applied to the topsurface of the spring contacts 15 to ensure that the etchant does notremove material from the top surface of the spring contacts 15 in casethe photosensitive material 17 protecting the top of the spring contacts15 fails during patterning of the spring contact 15.

Once the free portion 11 is freed from the insulating underlayer 13, thestress gradient Δσ/h causes the free portion 11 to bend up and away fromthe substrate 14. The stress gradient Δσ/h is still inherent in theanchor portion 12 and urges the anchor portion 12 to pull away from thesubstrate 14.

To decrease the chance of the anchor portion 12 pulling away from thesubstrate 14, the spring contact 15 can be annealed to relieve thestress in the anchor portion 12. This annealing process does not affectthe free portion 11 because, once the free portion 11 is released andallowed to bend up, no stress remains on the free portion 11 to berelieved by annealing. Thus, the free portion 11 remains curved up andaway from the substrate 14 after annealing.

Finally, FIG. 13 shows a layer of gold 19 plated over the outer surfaceof each spring contact 15. The layer of gold 19 is preferably used toreduce the resistance in the spring contacts 15, but can be replacedwith any other conductive material. Preferably, the gold layer 19 isplated on the spring contacts 15 using an electroless plating process.

Since the process for forming the spring contacts 15 is limited only bythe design rules of photolithographic patterning, many hundreds orthousands of spring contacts 15 can be formed closely together in arelatively small area on the substrate 14. The typical width w of thespring contact 15 is 10-100 μm. Therefore, the spring contacts 15 can beformed close together, at a spacing of approximately 10-20 μm. Thismakes the center-to-center distance between adjacent spring contacts 15approximately 20-120 μm, which is within or less than the typicalcenter-to-center distance between adjacent contact pads 3 on a standardsemiconductor chip 2.

To test the effectiveness of the spring contacts 15 in applicationssimilar to those found in solder-bump flip-chip bonding, a test array ofthe spring contacts 15 at a center-to-center spacing of 80 μm wasdeveloped as shown in FIG. 16. Four sets of arrays 20 of the springcontacts 15 were formed on a bottom substrate 21. Four correspondingarrays of linked contact pads 22 were formed on an upper substrate 23.The upper substrate 23 and the lower substrate 21 were brought togethersuch that the spring contacts 15 contacted a corresponding contact pad3. The resistance R was then measured across pairs of the spring contact15 leads.

FIG. 17 graphically depicts the measured resistance R for each springcontact pair in the test apparatus. The measured resistance R withineach array generally trends upward from left to right because of theincreased conductor length of the spring contacts 15 positioned to theright compared to the spring contacts 15 positioned to the left in eacharray. Most of the approximately 25-30 ohms of resistance measured foreach spring contact 15 pair is due to the length and geometry of theconductors extending between the spring contacts 15 and the resistance Rprobing points.

FIG. 18 shows the total resistance of the connection between a springcontact 15 and corresponding contact pad 3 with most of the resistance Rshown in FIG. 17 removed by using a 4-point probing geometry. As shownin FIG. 18, approximately 1.3 ohms of resistance is due to theconductors leading to the contact pad 3 and the spring contact 15.Approximately 0.2 ohms of resistance is due to the shape of the springcontact tip 30. The remaining resistance, approximately 0.1 ohms forb<80 μm, is the resistance at the interface between the contact pad 3and the spring contact tip 30.

In general, the resistance at the interface between the contact pad 3and the spring contact tip 30 decreases as the height b decreases. Asmentioned above, the reaction force F_(tip) that the spring contact tip30 exerts against the contact pad 3 increases as the contact pad 3pushes the spring contact tip 30 closer to the substrate 14. Theincreased reaction force F_(tip) causes the spring contact tip 30 tolocally deform at the contact pad 3, thereby increasing the contact areaand decreasing the resistance at the interface.

The shape of the spring contact tip 30 can take different forms,depending on the application. Since the spring contacts 15 arephotolithographically patterned, the spring contact tips 30 are easilyformed in a variety of shapes. FIG. 19 shows a spring contact tip 30having a flat end. The spring contact tip 30 shown in FIG. 20 has apointed end which concentrates the force F_(tip) exerted by the springcontact 15 at a single point on the contact pad 3. This pointed shapeaids the spring contact tip 30 when breaking through some oxides whichmay be present on the contact pads 3. FIGS. 21 and 22 show springcontact tips 30 having multiple points for applications where contactredundancy is required. FIG. 23 shows a spring contact tip 30 having adeformable tab. The deformable tab increases the contact area with thecontact pad 3, by deforming as shown in FIG. 24 when the spring contact15 forces the tip 30 against the contact pad 3.

Other methods are used to lower the contact resistance between thespring contact tip 30 and the contact pad 3. The spring contact tips 30can be ultrasonically scrubbed into the contact pads 3 to increase thearea of contact. Also, the spring contact tips 30 and the contact pads 3can be coated with solder 52 which is melted after the tips 30 and thecontact pads 3 are brought into contact. Melting the solder bonds thespring contacts 15 to the contact pads 3.

As mentioned above, since the production of the spring contacts 15 islimited only by the design rules of photolithographic patterning, thespring contacts 15 can be used to interconnect numerous different typesof devices. For example, FIG. 25 shows one preferred embodiment of theinvention. The spring contacts 15 are formed on the lower surface of thechip 2. The spring contacts 15 contact corresponding contact pads 3 onthe substrate 14. The adhesive 24 holds the chip 2 stationary withrespect to substrate 14. FIG. 26 shows the substrate 14 having aplurality of spring contacts 15 formed on the top surface of thesubstrate 14. The contact pads 3 formed on the lower surface of the chip2 are electrically connected to corresponding spring contacts 15 on thesubstrate 14. An adhesive 24 holds the chip 2 stationary relative to adust cover, or can, 25 covering the chip 2 and hermetically seals thedust cover 25 to the substrate 14. The dust cover 25 assures thatmoisture and other foreign substances do not corrode the spring contacts15 or the contact pads 3, or otherwise interfere with the electricalconnections between the individual spring contacts 15 and thecorresponding contact pads 3. Optional cooling fins 50 and the dustcover 25 provide a heat sink to cool the chip 2. FIG. 27 shows analternate form of the embodiment shown in FIG. 26. The adhesive 24 holdsthe chip 2 stationary to the substrate 14. No heat sink is provided bythe dust cover 25.

FIG. 28 shows an alternate embodiment of a connecting device forelectrically connecting two devices. A wafer 26 is shown having aplurality of spring contacts 15 formed on opposite sides of the wafer.Pairs of the spring contacts 15 on opposite sides of the wafer 26communicate with each other by way of vias etched in the wafer 26 andelectrically connect the contact pads 3 on both the chip 2 and thesubstrate 14. This embodiment of the invention allows processing of thechip 2 and the substrate 14 without risking damage to the springcontacts 15. The wafer 26 is used to interconnect the chip 2 and thesubstrate 14 only after all processing is completed on the chip 2 andthe substrate 14.

The spring contacts 15 are not limited to interconnecting a chip 2 to asubstrate 14 or circuit board. The spring contacts 15 are used equallywell to interconnect two chips 2, two circuit boards, or otherelectronic devices to each other. Two exemplary applications aremounting driver chips to visual displays and assembling multi-chipmodules (MCM's) for computers. Another alternative use for the springcontacts 15 is in probe cards. As discussed above, probe cards 7 areused to temporarily connect two devices, typically when one of thedevices is tested. Such testing is common in the semiconductor industry,where the probe cards 7 are used to test semiconductor chips while thechips are still part of a single-crystal silicon wafer.

FIG. 29 shows an embodiment of the invention where the probe card 27 hasan array of spring contacts 15 used in place of the standard probeneedles 8. The probe card 27 operates identically to the standard probecard 7 except for having spring contacts 15. The probe card 27 isaligned with the device 10 such that the spring contacts 15 compliantlycontact the corresponding contact pads 3 on the device 10. The device 10is then tested or communicated with by a testing device electricallyconnected to the probe card 27.

An example testing device is shown in FIG. 30 which is more thoroughlydescribed in the application U.S. patent application Ser. No.08/473,912, now abandoned, filed concurrently herewith. A displaypattern generator 40 communicates with driver chips 42 mounted on thetwo full-width probe cards 27. The probe cards 27 have the springcontacts 15 which contact associated addressing lines 43 formed on thedisplay plate 44. The addressing lines 43 communicate with displayelectrodes (not shown). Therefore, the display pattern generator 40 candrive the display electrodes to produce a matrix of electric potentialscorresponding to a test image. Sensors (not shown) on the sensor plate45 detect the matrix of electric potentials on the display electrodesand generate signals each corresponding to the electric potential. Thesignals are read out by scanner chips 46 mounted on the sensor plate 45.The test signal analyzer 41 receives the signals from the scanner chips46 and forms a sensed image corresponding to the signals. The testsignal analyzer 41 then compares the sensed image with the test imageoutput by the display pattern generator 40 to determine if the displayplate 44 and display electrodes are working properly.

Since producing a standard probe card 7 having probe needles 8 is laborintensive and time-consuming, standard probe cards 7 are not generallymade to contact all of the addressing lines 43 on the display plate 44.Therefore, testing of the display plate 44 must be done in sectionssince the probe cards 7 cannot accommodate the full width of theaddressing lines 43. In contrast, the probe card 27 made with springcontacts 15 can be made easily and inexpensively. Also, the probe cards27 having the spring contacts 15 can be made to any width and thereforecan test all of the data or address lines of an apparatus, such as thedisplay shown in FIG. 30, at one time.

In another example, wafer-scale testing and burning-in of chips 2 can beperformed by a single probe card 27 contacting all contact pads 3 of allchips 2 while the chips 2 are still part of a single semiconductorwafer. The probe card 27 can be a silicon wafer containingmicrocircuitry to distribute test signals to and from each chip 2 on thewafer under test. The test signals can be distributed either all at onceor sequentially to the chips 2.

While the invention has been described with reference to specificembodiments, the description of the specific embodiments is illustrativeonly and is not to be construed as limiting the scope of the invention.Various other modifications and changes may occur to those skilled inthe art without departing from the spirit and scope of the invention asset forth in the following claims.

What is claimed is:
 1. A spring contact, comprising: a substrate; and anelastic member composed of a single elastic material, the elastic memberhaving an inherent stress gradient and comprising an anchor portion anda free portion, the anchor portion fixed to the substrate; wherein theinherent stress gradient in the elastic member biases the free portionaway from the substrate, and the free portion makes sliding electricalcontact with a contact pad.
 2. A spring contact, comprising: asubstrate; and an elastic member composed of a single elastic material,the elastic member having an anchor portion and a free portion, theanchor portion fixed to the substrate; wherein the free portion isinitially fixed to the substrate to intentionally form an inherentstress gradient in the elastic member, the free portion is released fromthe substrate so the inherent stress gradient in the elastic memberbiases the free portion away from the substrate, and at least a portionof the elastic member is modified so that stresses in the anchor portionof the elastic member are less than stresses that biased the freeportion away from the substrate.
 3. The spring contact of claim 2,wherein the elastic member is formed from a plurality of layers of thesingle elastic material.
 4. The spring contact of claim 2, wherein thefree portion is bonded to a contact pad.
 5. The spring contact of claim2, wherein the free portion is bonded to a contact by solder.
 6. Thespring contact of claim 2, wherein the substrate to which the elasticmember is fixed comprises one of silicon, silicon dioxide and siliconnitride.
 7. The spring contact of claim 2, further comprising a secondmaterial fixed to at least a portion of the elastic member, the secondmaterial consisting of one of solder, gold, and a passivating material.8. The spring contact of claim 2, wherein the free portion has one of apointed tip and a flat end.
 9. The spring contact of claim 2, whereinthe elastic member is formed from a plurality of layers of the singleelastic material so that the elastic member has no discontinuousinterface between each of the layers.
 10. The spring contact of claim 2,wherein the inherent stress gradient is formed in the elastic member byone of including a reactive gas to a sputter deposition plasma,depositing at least a portion of the elastic member at an angle, andchanging a pressure of a sputter deposition plasma gas.
 11. The springcontact of claim 2, wherein the free portion is released from thesubstrate by heating a portion of the substrate.
 12. The spring contactof claim 2, further comprising at least one etching window formed in theelastic member.
 13. The spring contact of claim 2, wherein a tip of thereleased free portion is ultrasonically scrubbed into a contact pad. 14.The spring contact of claim 2, wherein a plurality of spring contactsare formed from the elastic member and each of the plurality of springcontacts is electrically contacted with a corresponding contact pad on adevice to be tested.
 15. The spring contact of claim 2, wherein theelastic member is packaged inside of a hennetically-sealed cover. 16.The spring contact of claim 2, wherein the elastic member is packagedalong with a device that is electrically connected to at least thereleased free portion inside of a hermetically-sealed cover.
 17. Thespring contact of claim 2, further comprising electronic circuitryformed in or on the substrate that communicates with the elastic member.18. The spring contact of claim 2, wherein portions of the elasticmaterial are selectively removed to form a plurality of individualspring contacts.
 19. The spring contact of claim 2, wherein the elasticmaterial is electrically conductive.
 20. A spring contact, comprising: asubstrate; and an elastic member composed of a single elastic material,the elastic member having an anchor portion and a free portion, theanchor portion fixed to the substrate; wherein the free portion isinitially fixed to the substrate to intentionally form an inherentstress gradient in the elastic member, and the free portion is releasedfrom the substrate by etching a layer of silicon nitride deposited onthe substrate so the inherent stress gradient in the elastic memberbiases the free portion away from the substrate.
 21. A spring contact,comprising: a substrate; and an elastic member composed of a singleelastic material, the elastic member having an anchor portion and a freeportion, the anchor portion fixed to the substrate; wherein the freeportion is initially fixed to the substrate by a release layer tointentionally form an inherent stress gradient in the elastic member,the free portion is released from the substrate by heating the substrateand the release layer so the inherent stress gradient in the elasticmember biases the free portion away from the substrate.
 22. A springcontact, comprising: a substrate; and an elastic member composed of asingle elastic material, the elastic member having an anchor portion anda free portion, the anchor portion fixed to the substrate; wherein thefree portion is initially fixed to the substrate to intentionally forman inherent stress gradient in the elastic member, the free portion isreleased from the substrate using application of a selective etchantthrough at least one etching window of a passivating layer so theinherent stress gradient in the elastic member biases the free portionaway from the substrate.
 23. A spring contact, comprising: a substrate;a contact pad; and an elastic member composed of a single elasticmaterial, the elastic member having an anchor portion fixed to thesubstrate and a free portion that contacts the contact pad; wherein thefree portion is initially fixed to the substrate to intentionally forman inherent stress gradient in the elastic member, the free portion isreleased from the substrate so the inherent stress gradient in theelastic member biases the free portion away from the substrate, and apart of the free portion of the elastic member is ultrasonicallyscrubbed into the contact pad.
 24. A spring contact, comprising: asubstrate; and an elastic member composed of a single elastic material,the elastic member having an anchor portion and a free portion, theanchor portion fixed to the substrate; wherein the free portion isinitially fixed to the substrate to intentionally form an inherentstress gradient in the elastic member, and the free portion is releasedfrom the substrate so the inherent stress gradient in the elastic memberbiases the free portion away from the substrate, and wherein theinherent stress gradient is formed in the elastic member by eitherincluding a reactive gas in a sputter deposition plasma or by changing apressure of a sputter deposition plasma gas.